While cristobalite has certain advantageous physical properties, it creates serious problems in refractory silica articles, particularly because of the extreme volume changes during the crystallographic alpha-beta inversion. The inversion can be catastrophic in high-density silica products, and, for this reason, the presence of crystalline silica in a silica glass has generally been considered undesirable. The semiconductor industry, for example, has heretofore believed that the silica used to make glass crucibles for Cz crystal-growing furnaces should be vitreous silica of high purity which is free of substantial amounts of cristobalite.
For similar reasons, the investment casting industry has avoided crystalline silica in silica glass cores. The presence of a few percent of alpha cristobalite in a full density glass core can cause an intolerable reduction in the modulus of rupture at 25.degree. C. and a substantial loss of thermal shock resistance without significantly improving the high temperature properties of the core.
Although it is well known that cristobalite has high-temperature sag resistance better than that of amorphous silica, those skilled in the art have been more concerned with the problems created by cristobalite than with its potential advantages as a refractory. One reason for lack of interest in cristobalite in the field of refractory silica glass is the difficulty of inducing crystallization in high-purity silica. The conversion of amorphous silica to cristobalite is difficult due to the very slow growth rate of the crystalline phase. For example, more than one hundred hours may be required to cause complete conversion of a silica crucible to the crystalline state. A further difficulty is that nucleation of cristobalite is only possible at free surfaces of the amorphous phase.
Because of the unknown and unpredictable nature of the crystallization process, those skilled in the art have been firmly convinced that cristobalite is an unreliable and undesirable material in high density refractory silica glass, particularly in glass cores and Cz crucibles.
Vitreous silica is also a preferred refractory material in the investment casting industry for manufacture of leachable cores. Although the internal air cooling passages in the blades and vanes of modern jet engines are usually formed by ceramic cores rather than glass cores, the smaller compressor or turbine airfoils having cooling passages with a diameter for 0.3 mm to 1.5 mm preclude the use of ceramic cores. In such small diameters, ceramic cores have no utility. Only silica glass provides the strength required for wax injection and metal casting when using the smaller cores.
Full density vitreous silica core made by glass drawing have been used for many years in the investment casting industry. They are used, for example, in a conventional investment casting process in which the shell mold and the core are preheated to a high temperature, such as 1000.degree. C., and a molten alloy at a temperature of 1450.degree. C. to 1550.degree. C., or higher is poured into the mold cavity. The process is usually conducted under a vacuum and produces cast metal structures having a multiplicity of fine equiaxed grains and referred to as "equiaxed" castings. The dimensional stability problem becomes more severe with vitreous silica cores as the metal pouring temperature increases. However, the use of full-density silica glass cores in equiaxed casting processes has been very successful. The glass cores are generally well suited to such processes because they have good strength and excellent thermal shock resistance.
In recent years, there has been a trend toward use of the directional solidification (D.S) casting process in the manufacture of turbine airfoil components which produces a columnar grain structure extending from one end of the part to the other. The D.S. castings have high temperature properties far superior to those of their equiaxed counterparts. In the D.S. casting process, the mold, which is open at the bottom end, is placed on a copper chill plate and the metal casting is progressively solidified and gradually cooled as by gradually lowering the chill plate away from the heating zone (see U.S. Pat. Nos. 3,700,023 and 4,093,017). In this process, the mold is usually preheated to a temperature of from 1350.degree. C. to 1500.degree. C. or higher and a molten superalloy is poured into the mold at a temperature above 1500.degree. C.
The D.S. casting subjects the refractory molds and cores to much higher temperatures for longer times than the equiaxed casting process. In a typical D.S. process, the mold can be subjected to a temperature above 1450.degree. C. for one-half hour to one hour or more. Under these conditions, a vitreous silica core, which is a glass, is subject to viscous flow and will distort and move or sag. The core cannot support its own weight. At a lower temperature, such as 1350.degree. C., the viscosity of the glass is much higher and little distortion will occur unless a substantial external load is applied. Because of the lower temperatures employed in equiaxed casting, full density vitreous silica cores are able to function satisfactorily in an equiaxed casting process even though they are unacceptable for D.S. casting.
In the case of porous ceramic cores, improved resistance to deformation during the D.S. casting process can be achieved by devitrifying the core and converting a major portion of the vitreous silica to cristobalite as disclosed in U.S. Pat. No. 4,093,017. Unfortunately, the process disclosed in that patent is not applicable to high-density glass cores (e.g., because of the catastrophic crystallographic alpha-beta inversion problem). A relatively small percentage of crystalline silica in a high density glass core will crack or shatter the core because of the large volume change which occurs when the silica changes from the alpha to the beta form or vice versa (see U.S. Pat. No. 3,540,519).
The crystallographic alpha-beta inversion occurs whenever crystalline silica is heated or cooled through the temperature range of from about 180.degree. C. to 250.degree. C. The problem is particularly severe with respect to cores used in precision investment casting.
There are a number of reasons why cristobalite has been considered intolerable in glass crucibles used for Cz crystal-growing furnaces. The extreme vertical temperature gradients in such furnaces and the severe operating conditions during crystal growth magnify the problems created by cristobalite. Experience has demonstrated that minute amounts of crystalline silica cause crucible deterioration and failure of the vitreous silica crucibles and that crucibles entirely free of cristobalite avoid such problems.
During the last decade, the semiconductor industry has insisted that slip-cast crucibles used in Cz crystal-growing furnaces be free of cristobalite and that such crucibles be sintered at a temperature above 1750.degree. C. long enough to eliminate all of the crystalline silica.